Blender to Unity animation | Volume Preservation
From Cygon4's post (Feb 16, 2016)


  [...] the "Preserve Volume" option (= dual quaternion skinning) does not exist in Unity.


You have to find a weights only method to preserve your volume. A common practice is to use a helper bone at angled areas.


Create the chain of bone1 and bone2.
Create bone1_helper and parent it to bone1.
Constrain the rotation from bone2 to bone1_helper, use local space and an influence of 0.5.



The gif showcases the base model, then Preserve Volume option, then the helper bone option. 

Bake the animations with visual transform before exporting.